In Fischer-Tropsch synthesis, synthesis gas comprising carbon monoxide and hydrogen is converted to mostly hydrocarbons and water over a heterogeneous catalyst. Although various metals are known to catalyse Fischer-Tropsch synthesis reactions, only catalysts comprising iron (Fe) or cobalt (Co) have found large scale commercial application to date.
Fischer-Tropsch synthesis can be applied in a variety of reactors, as discussed, for example, in the book entitled “Fischer-Tropsch Technology”, Dry and Steynberg (Eds.), Stud. Surf. Sci. Catal., Vol. 152, October 2004 (Elsevier). The reactors can broadly be divided into two groups, namely stationary bed reactors and moving bed reactors. In stationary bed reactors, the catalyst bed is typically fixed in one position. Examples of stationary bed reactors include multi-tubular fixed bed reactors and micro-channel reactors. In moving bed reactors, catalyst particles move around freely inside the reactor. Examples of moving bed reactors include two-phase fluidised bed and three-phase slurry bed reactors.
Fischer-Tropsch catalysts deactivate under synthesis conditions (i.e. conditions of elevated temperatures and pressures) for a variety of reasons, including, for example, by poisoning due to nitrogen- or sulphur-containing compounds present in the synthesis gas, sintering of metal crystallites in the catalyst itself and coke deposition on active catalyst sites. Water is a by-product of the Fischer-Tropsch reaction and is well known to contribute to the deactivation of the catalyst. The rate of deactivation of the catalyst is a function of both the catalyst itself, for example catalyst composition, method of preparation, etc., and the process conditions under which it is operated and to which it is exposed, for example the level or concentration of poisons in synthesis feed gas, reactor operating temperature, reagent partial pressures, conversion, etc.
Certain catalyst deactivation mechanisms are readily reversible, for example by subjecting the catalyst to a treatment involving contacting the catalyst with a reducing gas such as a hydrogen containing gas. Other deactivation mechanisms may only be reversed by more severe treatments, for example by treatment processes comprising multiple steps which would typically involve steps of reducing a wax content of the catalyst (for example by settling out the catalyst from a catalyst slurry (i.e. a catalyst-containing slurry), followed by a solvent wash or a hydrogen treatment), exposure of the catalyst to an oxygen containing gas inter alia to burn off or oxidise carbonaceous deposits on the catalyst, and finally a reduction step in which the catalyst is activated for use in the Fischer-Tropsch synthesis, for example by reduction with a hydrogen containing gas.
Since a hydrogen rejuvenation treatment is only able to reverse a limited number of the catalyst deactivation mechanisms, it is normally less efficient in restoring catalyst activity than an oxidative regeneration treatment, especially for older catalysts. Over time, due to an accumulation of deactivation effects that are not reversible by a rejuvenation treatment, the catalyst will become less and less active and ultimately unfit for further use if only reactivation by way of rejuvenation treatment is applied. On the other hand, a regeneration (oxidative) treatment is usually more aggressive than a rejuvenation treatment (e.g. by hydrogen reduction), since it exposes the catalyst to much higher temperatures, often in the presence of steam formed in the oxidation step.
Amongst others, the hydrothermal conditions to which a catalyst is exposed in a regeneration treatment could lead to a deterioration of the catalyst over time, often limiting the number of regeneration treatments to which a catalyst can be sensibly exposed. For example, as a catalyst is exposed to an increasing number of reactivation treatments, reactivation becomes increasingly less effective in restoring catalyst performance. This is mainly due to the cumulative negative effects of multiple reactivation treatments on catalyst integrity and activity. For instance, Shell has reported that the overall catalyst lifetime of their commercial fixed bed Fischer-Tropsch catalyst can be extended to five years by performing an annual regeneration treatment (A. Hoek, L. B. J. M. Kersten, “The Shell Middle Distillate Synthesis Process: technology, products and perspective”, Stud. Surf. Sci Catal., Vol. 147 (Nat. Gas. Cony. VII), pp. 25-28). This implies that the Shell fixed-bed Fischer-Tropsch catalyst can be subjected to four regeneration cycles before it becomes unfit for further use, after which the fixed-bed Fischer-Tropsch reactor must be reloaded with a fresh batch of catalyst in order that a new production cycle can be initiated.
When a Fischer-Tropsch synthesis process is operated in a fixed bed reactor, it is not always convenient to remove the catalyst from the reactor for purposes of reactivation. The reactivation process to recover some or all of the lost activity of the catalyst is then often rather effected in situ. A disadvantage of in situ reactivation is that the operation of the Fischer-Tropsch synthesis process has to be suspended or interrupted before the reactivation can be performed, i.e. the reactivation is performed off-line. Depending on the reactivation process, this can result in a lengthy interruption of Fisher-Tropsch synthesis. For example, delays may be caused by heating up or cooling down the catalyst bed during or between steps of the reactivation process or purging of the Fischer-Tropsch reactor to avoid the possibility of forming explosive gas mixtures in case where an oxidative step is applied in the reactivation process. Typically, the full catalyst inventory is reactivated during an off-line in situ reactivation process, meaning that all catalyst particles in the Fischer-Tropsch reactor would be subjected to an equal number of reactivation treatments.
Moving bed reactors have the advantage that catalyst can usually be withdrawn or added during normal operation without significantly affecting the Fischer-Tropsch synthesis reactions. This affords an operator the opportunity of withdrawing a portion of the catalyst inventory from a Fisher-Tropsch reactor, subjecting it to a reactivation treatment in order to restore some or all of the catalyst activity and returning the reactivated catalyst to the Fischer-Tropsch reactor for further use, while keeping the Fischer-Tropsch reactor on-line. Various methods for the on-line withdrawal and reactivation of Fischer-Tropsch catalyst have been suggested in the prior art.
In U.S. Pat. No. 5,260,239 a reactor arrangement that allows for the continuous circulation of catalyst slurry between a slurry phase Fischer-Tropsch reactor and a slurry phase hydrogen rejuvenation reactor by using a system of downcomers is disclosed. Catalyst slurry containing partially deactivated catalyst is fed under flow of gravity from the Fischer-Tropsch reactor to the rejuvenation vessel where it is exposed to hydrogen in order to recover some of the lost activity, while slurry containing rejuvenated catalyst is cycled back to the Fischer-Tropsch reactor.
U.S. Pat. No. 6,900,151 discloses a slurry phase Fischer-Tropsch process which involves the regeneration of catalyst. Slurry containing catalyst is withdrawn from the Fischer-Tropsch reactor and regenerated via an oxidative treatment, leaving the active metals in the oxide phase. The slurry Fischer-Tropsch reactor, which is supplied with an in situ hydrogen rejuvenation means, receives the catalyst in unreduced (oxidised) form, whereafter it is reduced in situ to the metallic state by contact with hydrogen.
In U.S. Pat. No. 6,900,151, the treatment of a deactivated catalyst only with a reducing gas in order to increase its activity is typically called rejuvenation, whereas a treatment involving at least an oxidative step is called regeneration. It will be apparent from an assessment of the art that in other instances regeneration may refer to any treatment of a deactivated catalyst in order to recover some or all of its activity. A clear definition of the relevant technical terms is essential for a proper understanding of the present invention.
In this specification, hereinafter: (i) the term “reactivation” should be understood to mean any method of treating a partially deactivated catalyst in order to recover at least some of its lost activity and thus includes “regeneration” and “rejuvenation”, so that a reactivated catalyst can be a regenerated catalyst, or a rejuvenated catalyst, or a catalyst that has been both regenerated and rejuvenated; (ii) the term “rejuvenation” should be understood to mean a treatment of a deactivated catalyst by contact with a reducing agent, for example by contact with a hydrogen containing gas, but without contact with an oxidising agent, in order to recover at least some of its lost activity; and (iii) the term “regeneration” should be understood to mean a treatment of a deactivated catalyst by contact with an oxidising agent, for example an oxygen containing gas, in at least one step of a reactivation treatment in order to recover at least some of its lost activity.
Furthermore, the term “fresh catalyst” should be understood to mean a newly manufactured or never before used catalyst, i.e. a catalyst that has never before been used to produce Fischer-Tropsch products under synthesis conditions, whereas the term “reactivated catalyst” should be understood to mean a used catalyst that has been subjected to reactivation.
WO 2001/036352 discloses a Fischer-Tropsch process in which catalyst is regenerated by means of a steam treatment. WO 2001/036352 also teaches cycling of catalyst between the Fischer-Tropsch synthesis process and a regeneration process on a continuous basis.
U.S. Pat. No. 6,201,030 describes a slurry Fischer-Tropsch reactor with two regenerators. In the process of U.S. Pat. No. 6,201,030 a deactivated catalyst is unloaded to one regenerator whilst regenerated catalyst is returned to the slurry Fischer-Tropsch reactor from another regenerator.
US 2005/0124706 discloses a process of cycling catalyst batches between a slurry phase Fischer-Tropsch reactor and a regeneration process by applying a pressure swing condition to a catalyst.
US 2010/0240777 discloses a slurry phase Fischer-Tropsch process in which the activity of a deactivated catalyst is restored by subjecting the catalyst to a hydrogen treatment. The exposure of the catalyst to hydrogen is effected either inside the synthesis reactor or in an external circulation stream of catalyst slurry. US 2010/0240777 terms contact with hydrogen a “regeneration” of the catalyst, but since this only entails exposing the catalyst to a reducing gas, it is rather a rejuvenation in terms of the defined terminology in the present specification.
WO 2003/064356 and WO 2003/064034 both describe the removal of slurry containing deactivated catalyst from a slurry reactor, subjecting it to a regeneration treatment and returning the catalyst to the reactor. Provision is made for the removal of fine particles from the withdrawn slurry. Preferably, the removal of fine particles is done as part of the regeneration process. Catalyst fines are undesirable for slurry reactor operations as they can lead to operational problems. Both WO 2003/064356 and WO 2003/064034 thus teach that the regeneration procedure can advantageously also be used for the reduction of undesirable fines inside the slurry phase Fischer-Tropsch reactor.
WO 2012/022942 also describes a slurry Fischer-Tropsch process in which batches of slurry containing deactivated catalyst are removed from a Fischer-Tropsch synthesis reactor and subjected to a regeneration treatment. Preferably, undesirable catalyst fines are removed from the regenerated catalyst before it is reloaded back into the Fischer-Tropsch synthesis reactor in order to mitigate the adverse effects of fine particles on slurry reactor operation.
WO 2012/056346 discloses a method of operating a process for catalytically converting one or more reactants to one or more products using a fluid bed reactor (e.g. a three-phase slurry bed reactor) containing a catalyst (e.g. a Fischer-Tropsch catalyst) which deactivates over time. The method includes adding a catalyst which has the tendency to increase the conversion rate of one or more reactants into the reactor, and reducing the operating temperature of the reactor to counteract to at least some extent the effect of the added catalyst on the conversion rate of the one or more reactants.
Methods of removing catalyst from a Fischer-Tropsch synthesis reactor, subjecting the removed catalyst to a treatment in order to regain some or all of its activity and returning the reactivated catalyst to the Fischer-Tropsch synthesis reactor are therefore known in the prior art. Additionally, the art teaches that the reactivation step can conveniently also be used to remove undesirable catalyst fines, generated either during the Fischer-Tropsch synthesis process or during the reactivation process itself, from a slurry reactor.
In a moving-bed reactor, such as a three-phase slurry bubble column Fischer-Tropsch synthesis reactor, the catalyst particles can move around freely and are essentially well mixed. It follows that the catalyst particles withdrawn from such a Fischer-Tropsch synthesis reactor for reactivation is a random sample of catalyst particles present therein. Therefore, in a Fischer-Tropsch reactor in which on-line reactivation of catalyst is employed, a distribution of catalyst particles with different activities will be present depending on the reactivation history of each catalyst particle. Furthermore, a distribution of catalyst particles that has been exposed to varying numbers of reactivation treatments will develop over time, i.e. some catalyst particles might have undergone a large number of reactivation treatments, whereas other catalyst particles might not have been reactivated at all. This is particularly important where the reactivation treatment includes regeneration. Additionally, Fischer-Tropsch synthesis reactor performance will increasingly deteriorate as a portion of the catalyst inventory inside the Fischer-Tropsch reactor that is no longer suitably reactivated by the reactivation treatment continuously increases over time. Eventually this drop in Fischer-Tropsch synthesis reactor performance will necessitate a discarding of the whole catalyst inventory and restarting with fresh catalyst. This in turn requires interruption of plant operation and is therefore undesirable. The prior art has failed to address these issues.
A method of synthesising Fisher-Tropsch products which employs catalyst reactivation and which allows for extended, stable on-line operation would be an advantage.